1. Field of the Invention
This invention relates to mobile cranes for use on offshore oil well platforms, and to the utilization of such cranes in the erection of workover rigs on such platforms.
2. Description of the Prior Art
Offshore oil and gas wells are often completed at the dock of a platform which is supported above the water by means of a plurality of legs. Usually each platform will have a plurality of wells completed thereon, ranging from a few wells up to 40 or more. From time to time after the completion of a well, it is necessary to bring a workover rig to the platform to perform certain operations on the well. One form of workover rig suitable for this purpose is disclosed in applicant's U.S. Pat. No. 3,802,137, issued Apr. 9, 1974.
Such a workover rig is designed to rest on the capping beams of the platform which may be from 25 feet to 55 feet apart and which extend from one side of the platform to the other. Such beams are extremely heavy and strong, having sufficient strength to support the loads imposed by the workover Fig. The portion of the deck between the capping beams and extending outwardly therefrom usually has insufficient strength to support such loads. In addition, those outwardly extending portions of the deck usually have mounted thereon various tanks, pumps and other production equipment which may be necessary in connection with the production of oil or gas from the well and for the transporting of the oil or gas from the well to the shore.
Such platforms also normally are equipped with a small affixed crane which is positioned for on loading and off loading of equipment. However, such platform cranes have been inadequate for placing a workover rig on the platform, since the platform crane is in a fixed position and has insufficient capacity at a long enough distance to allow it to place the workover rig in position for operating on all of the wells of the platform. Thus, it has heretofore been necessary to use a derrick barge to erect the workover rig on the platform and to remove it from the platform after the work on each platform is completed. Since derrick barges are quite expensive, it often cost from $60,000 to $200,000 just to move a rig from one platform to another. Then when the rig's work on that platform was finished, it was necessary to bring the derrick barge back to remove the rig from that platform and move it to and erect it on another platform.